Interactions of phenolic acid derivatives with interfacial proteins and their impact on emulsion stability

Processed foods are often emulsions, which are thermodynamically unstable systems. Proteins such as β-lactoglobulin (β-Lg) can stabilize the oil-water interface in emulsions due to their amphiphilic, film-forming, and electrostatic properties. They initially adsorb at the interface and undergo conformational changes. At the molecular level of an emulsion, intermolecular interactions between interfacial proteins enable the formation of a viscoelastic interfacial film. This, along with electrostatic effects, stabilizes the emulsion at the macrostructural level, resulting in both physical and chemical emulsion stability. In food-based emulsions, proteins also interact with other components such as phenolic compounds. These interactions can compete with the intermolecular protein interactions within the interfacial film and influence the protein conformation, leading to changes in emulsion stability. However, studies show that these interactions can either increase or decrease the physical and chemical stability of an emulsion. In fact, studies focusing on interactions between phenolic compounds and proteins in aqueous environments indicate that these interactions are significantly affected by intrinsic factors, and the ambient conditions which could explain these opposing findings. Intrinsic factors primarily include the structural properties of the phenolic compounds and proteins, while ambient conditions describe parameters such as the pH. For emulsion systems, it is not yet systematically understood how these factors affect the interactions between phenolic compounds and interfacial proteins at the molecular level and how the changes reflect at the macrostructural level. This thesis aims to clarify the effects of the factors interaction place, pH, and chemical structure of phenolic compounds on the interactions with β-Lg at the molecular level and to characterize their impact on the emulsion stability at the macrostructural level. The interaction place in the emulsion was varied because β-Lg exists in a globular conformation in the aqueous phase before adsorption and then partially unfolds at the interface, affecting protein conformation and sterically accessible binding sites. The pH influences the protonation degree of molecules and thus, the electrostatic effects, which in turn affect the interactions between phenolic compounds and β-Lg at the interface. The chemical structure of phenolic compounds influences their partitioning behavior between oil, interface, and water, whereby phenolic compounds can interact with the interfacial protein from either the oil phase or the aqueous phase. For this purpose, phenolic acid derivatives (PCD) based on the structure of caffeic acid were selected, with systematically varied substituents. The results of this thesis showed that the interaction of PCD and β-Lg in the aqueous phase leads to an increase in the adsorption and film formation time of the protein at the oil-water interface, presumably due to PCD-induced conformational changes that may result in reduced surface hydrophobicity of β-Lg. When the interaction place is the interface, interfacial adsorption and film formation are not affected. However, partially unfolded β-Lg exhibits more binding sites for PCD than globular β-Lg in the aqueous phase. Hence, the PCD-β-Lg interactions compete with the intermolecular protein interactions, leading to a reduction in interfacial viscoelasticity. The low electrostatic repulsion between β-Lg and PCD at pH 6 favors the co-adsorption of PCD at the interface and partially displaced the adsorbed protein. In contrast, increasing electrostatic repulsion between PCD and β-Lg at pH 9, prevented the co-adsorption of PCD and the displacement of interfacial proteins. The increasing reactivity of PCD at pH 9 favors the formation of covalent bonds between PCD and β-Lg, although PCD preferentially react with each other to form polymerization products. At the macrostructural level, the physical emulsion stability increased with increased electrostatic repulsion at pH 9, leading to stronger repulsion and therefore, less coalescence of the droplets compared to pH 6. The chemical emulsion stability also increases at pH 9, as PCD exhibit antioxidant properties favored by increased reactivity and stronger complex formation through phenolic hydroxyl groups with pro-oxidative transition metals. At pH 6, the pro-oxidative nature of less reactive PCD predominates due to weak complexation through carboxyl groups. Regarding the chemical structure, it was found that extended π-electron systems of PCD and an increasing molecular size contribute to the cross-linking of side chains of adsorbed proteins on different droplets, protecting the emulsion from coalescence by forming a network. Polar substituents shift the distribution of PCD towards the aqueous phase, where radicals can only be scavenged within the phase where the PCD are localized. This thesis shows how the interactions of PCD with interfacial protein films can be systematically affected by intrinsic factors and ambient conditions at the molecular level and explain the resulting varying macrostructural properties of an emulsions. The findings thereby extend the understanding of PCD-protein interactions in aqueous systems to multiphase food systems like emulsions. Consequently, this thesis contributes to a systematic comprehension of the behavior of multiphase food systems.